The present invention relates to a radio communication system and a base station in which a plurality of base stations transmit the same communication information such as the same voice and sound and data at same timing to an access terminal and the access terminal combines the communication information with each other to restore the original data stream.
In the field of communications in recent years, the broadband communication has been broadly developed, routers have been technically advanced, and a need of users to receive, for example, streaming video images having a large capacity or including a large amount of data is increasing. In association therewith, attention has been attracted to a multicast communication technique to broadcast data as well as voice and sound having a large capacity using a few communication resources as compared with a unicast communication technique. “Multicast” is a technique to simultaneously transmit the same communication information such as the same data and/or voice and sound to a plurality of user terminals. “Unicast” is a technique to conduct communication with each user terminal with a one-to-one correspondence established therebetween.
Therefore, when plural access terminals have received the same voice or data by using the multicast, the traffic on the communication can be advantageously reduced when compared with the case using the unicast.
Also, in the field of radio communication such as Evolution Data Only (1xEVDO) using, for example, a Code Division Multiple Access (CDMA), there has been discussed a communication technique in which to improve efficiency of use of radio bands, one data stream can be received by a plurality of access terminals. Heretofore, a physical channel is assigned to each access terminal to establish radio connection for communication therebetween. That is, one-to-one unicast communication is carried out. By assigning one particular channel one the physical channels as a multicast channel, one data stream is sent to a plurality of access terminals. This improves efficiency of use of radio bands.
For efficient use of frequencies, each of the base stations constituting a radio communication system includes a plurality of sectors. An electric wave receiving state of the access terminal varies depending on a wave propagation environment between the access terminal and the base stations. In the unicast, one-to-one communication is conducted between an access terminal and a base station (sector). Therefore, the base station (sector) can control the communication. For example, the base station increases, for each access terminal, a data rate of a data stream when the wave propagation state is good for the access terminal and decreases the data rate thereof when the wave propagation state is unsuitable or is not good for the access terminal. In the multicast, it is required for the access terminal to receive a data stream sent from the base station (sector) at a particular data rate stipulated for the multicast. Therefore, since the data rate of the data stream received by the access terminal varies depending on the wave propagation environment, when the data stream is sent from the base station (sector) at a high data rate in the multicast, an access terminal at a position in a deteriorated or bad wave propagation environment cannot receive the data stream. When the data rate is low, throughput of the communication is reduced.
Reception sensitivity of a signal received by the access terminal depends on a ratio between an actual data signal level of the received data signal and a noise signal level thereof. Therefore, to enable the multicast data stream to be received by as many access terminals as possible, it is only necessary to increase the ratio of the actual data signal level to the noise signal level.
However, since a base station (sector) in a best wave propagation environment is selected for communication in the conventional unicast communication, substantially all electric waves sent from base stations (sectors) not selected as communication targets become interference noise. The 3rd Generation Partnership Project 2 (3GPP2) C.S0054 Version 1.0 describes a method in which a plurality of base stations (sectors) transmit at same timing the same data stream using the same frequency to the access terminal. The access terminal combines data signals of the data streams with each other to restore the original data stream to thereby increases the ratio of the actual data signal level to the noise signal level. As a result, an increased number of access terminals can receive data streams multicast at a high data rate. In a technique called Soft Combine, the same data streams sent from a plurality of base stations (sectors) at same timing are combined with each other and restored into the original data stream by the access terminal. This increases the ratio of the actual data signal level to the noise signal level of the signal received by the access terminal to thereby improve data stream reception quality.
Timing to transmit the data stream is sent from the base stations (sectors) to the access terminals at a fixed interval of time.